Lack of understanding of the true nature of happiness, it seems to me, is the principal reason why people inflict sufferings on others. They think either that the other's pain may somehow be a cause of happiness for themselves or that their own happiness is more important, regardless of what pain it may cause. But this is shortsighted: no one truly benefits from causing harm to another sentient being. Whatever immediate advantage is gained at the expense of someone else is shortlived. In the long run, causing others misery and infringing their rights to peace and happiness result in anxiety, fear, and suspicion within oneself. Such feelings undermine the peace of mind and contentment which are the marks of happiness.
True happiness comes not from a limited concern for one's own well-being, or that of those one feels close to, but from developing love and compassion for all sentient beings. Here, love means wishing that all sentient beings should find happiness, and compassion means wishing that they should all be free of suffering. The development of this attitude gives rise to a sense of openness and trust that provides the basis for peace.
--The Dalai Lama, from The Dalai Lama: A Policy of Kindness, edited by Sidney Piburn
from Everyday Mind, edited by Jean Smith, a Tricycle book